1. Field of the Invention
The present invention relates to apparatuses and methods for evaluating subsurface formations in wellbore operations. More particularly, the present invention relates to wellbore systems for performing formation evaluation, such as testing and/or sampling, using a downhole tool positionable in a wellbore penetrating a subterranean formation.
2. Background of the Related Art
Wellbores are drilled to locate and produce hydrocarbons. A downhole drilling tool with a bit at an end thereof is advanced into the ground to form a wellbore. As the drilling tool is advanced, a drilling mud is pumped from a surface mud pit, through the drilling tool and out the drill bit to cool the drilling tool and carry away cuttings. The fluid exits the drill bit and flows back up to the surface for recirculation through the tool. The drilling mud is also used to form a mudcake to line the wellbore.
During the drilling operation, it is desirable to perform various evaluations of the formations penetrated by the wellbore. In some cases, the drilling tool may be provided with devices to test and/or sample the surrounding formation. In some cases, the drilling tool may be removed and a wireline tool may be deployed into the wellbore to test and/or sample the formation. In other cases, the drilling tool may be used to perform the testing or sampling. These samples or tests may be used, for example, to locate valuable hydrocarbons.
Formation evaluation often requires that fluid from the formation be drawn into the downhole tool for testing and/or sampling. Various fluid communication devices, such as probes, are extended from the downhole tool to establish fluid communication with the formation surrounding the wellbore and to draw fluid into the downhole tool. A typical probe is a circular element extended from the downhole tool and positioned against the sidewall of the wellbore. A rubber packer at the end of the probe is used to create a seal with the wellbore sidewall. Another device used to form a seal with the wellbore sidewall is referred to as a dual packer. With a dual packer, two elastomeric rings expand radially about the tool to isolate a portion of the wellbore therebetween. The rings form a seal with the wellbore wall and permit fluid to be drawn into the isolated portion of the wellbore and into an inlet in the downhole tool.
The mudcake lining the wellbore is often useful in assisting the probe and/or dual packers in making the seal with the wellbore wall. Once the seal is made, fluid from the formation is drawn into the downhole tool through an inlet by lowering the pressure in the downhole tool. Examples of fluid communication devices, such as probes and/or packers, used in downhole tools are described in U.S. Pat. Nos. 6,301,959; 4,860,581; 4,936,139; 6,585,045; 6,609,568 and 6,719,049 and US Patent Application No. 2004/0000433.
Once the fluid enters the downhole tool, it may be tested, collected in a sample chamber and/or discharged into the wellbore. Techniques currently exist for drawing fluid into the downhole tool and/or performing various downhole operations, such as downhole measurements, pretests and/or sample collection of fluids that enter the downhole tool. Examples of such techniques may be found in U.S. Pat. Nos. 4,860,581; 4,936,139; 5,303,775; 5,934,374; 6,745,835 3,254,531; 3,859,851; 5,184,508; 6,467,544; 6,659,177; 6,688,390; 6,769,487; 2003/042021; 2004/0216874; and 2005/0150287.
In some cases, the wellbore environment may be exposed to extremely high temperatures and/or pressures which may cause electronics and other tool components to fail. Techniques for cooling instrumentation, such as electronic circuits, in a downhole tool are described, for example, in U.S. Pat. Nos. 5,701,751; 6,769,487 and US 2005/0097911.
Despite the development and advancement of formation evaluation techniques in wellbore operations, there remains a need to provide a formation evaluation system capable of operating in even the harshest wellbore environments having extreme temperatures and/or pressures. It is desirable that such a system be capable of efficiently cooling electronics in the downhole tool. It is further desirable that such a system eliminate, reduce and/or protect components that are subject to failure in harsh wellbore conditions. Such a system preferably provides one or more of the following among others: a fluid flow system that does not require a pump to draw fluid into the tool, consolidated electronics for efficient cooling, gauges (such as formation fluid sensors) located with or near the consolidated electronics for cooling, pressure balanced sample and/or dump chambers and increased cooling efficiency.